Cutting Tools Episode 1 Part 8 pot

Taper shank Tang Tang drive Neck Shank diameter Axis Straight shank Land width Point angle Lip relief angle Helix angle Drill diameter Clearance Diameter Body Diameter Clearance Chisel E

8.1 Introduction Drilling is the process most commonly associated with producing machined holes Although many other processes contribute to the production of holes, including boring, reaming, broaching, and internal grinding, drilling accounts for the major-ity of holes produced in the machine shop This

is because drilling is a simple, quick, and eco-nomical method of hole production The other methods are used principally for more accurate, smoother, larger holes They are often used after

a drill has already made the pilot hole

Drilling is one of the most complex machining processes The chief characteristic that distin-guishes it from other machining operations is the combined cutting and extrusion of metal at the chisel edge in the center of the drill The high thrust force caused by the feeding motion first extrudes metal under the chisel edge Then it tends to shear under the action of a negative rake angle tool Drilling of a single hole is shown in Figure 8.1 and high production drilling of a plate component is shown in Figure 8.2

Chapter 8 Drills & Drilling Operations

Upcoming Chapters

Metal Removal

Cutting-Tool Materials

Metal Removal Methods

Machinability of Metals

Single Point Machining

Turning Tools and Operations

Turning Methods and Machines

Grooving and Threading

Shaping and Planing

Hole Making Processes

Drills and Drilling Operations

Drilling Methods and Machines

Boring Operations and Machines

Reaming and Tapping

Multi Point Machining

Milling Cutters and Operations

Milling Methods and Machines

Broaches and Broaching

Saws and Sawing

Finishing Processes

Grinding Wheels and Operations

Grinding Methods and Machines

Lapping and Honing

George Schneider, Jr CMfgE

Professor Emeritus

Engineering Technology

Lawrence Technological University

Former Chairman

Detroit Chapter ONE

Society of Manufacturing Engineers

Former President

International Excutive Board

Society of Carbide & Tool Engineers

Lawrence Tech Univ.: http://www.ltu.edu

Prentice Hall: http://www.prenhall.com

FIGURE 8.2: Holes can be drilled individually as shown in Figure 8.1, or many holes can be drilled at the same time as shown here (Courtesy Sandvik Coromant Co.)

FIGURE 8.1: Drilling accounts for the majority of holes produced in industry today (Courtesy Valenite Inc.)

The cutting action along the lips of

the drill is not unlike that in other

machining processes Due to variable

rake angle and inclination, however,

there are differences in the cutting

action at various radii on the cutting

edges This is complicated by the

con-straint of the whole chip on the chip

flow at any single point along the lip

Still, the metal removing action is true

cutting, and the problems of variable

geometry and constraint are present, but

because it is such a small portion of the

total drilling operation, it is not a

distin-guishing characteristic of the process

Many of the drills discussed in this

chapter are shown in Figures 8.3

The machine settings used in drilling

reveal some important features of this

hole producing operation Depth of cut,

a fundamental dimension in other

cut-ting processes, corresponds most

close-ly to the drill radius The undeformed

chip width is equivalent to the length of

the drill lip, which depends on the point

angle as well as the drill size For a

given set-up, the undeformed chip

width is constant in drilling The feed

dimension specified for drilling is the

feed per revolution of the spindle A

more fundamental quantity is the feed

per lip For the common two-flute drill,

it is half the feed per revolution The

undeformed chip thickness differs from

the feed per lip depending on the point

angle

The spindle speed is constant for any

one operation, while the cutting speed

varies all along the cutting edge

Cutting speed is normally computed for

the outside diameter At the center of

the chisel edge the cutting speed is zero;

at any point on the lip it is proportional

to the radius of that point This varia-tion in cutting speed along the cutting edges is an important characteristic of drilling

Once the drill engages the workpiece, the contact is continuous until the drill breaks through the bottom of the part or

is withdrawn from the hole In this respect, drilling resembles turning and

is unlike milling Continuous cutting means that steady forces and tempera-tures may be expected shortly after con-tact between the drill and the work-piece

8.2 Drill Nomenclature The most important type of drill is the twist drill The important nomenclature listed below and illustrated in Figure 8.4 applies specifically to these tools

Drill: A drill is an end-cutting tool

for producing holes It has one or more cutting edges, and flutes to allow fluids

to enter and chips to be ejected The drill is composed of a shank, body, and

point

Shank: The shank is the

part of the drill that is held and driven It may be straight

or tapered Smaller diameter drills normally have straight shanks Larger drills have shanks ground with a taper and a tang to insure accurate alignment and positive drive

Tang: The tang is a

flat-tened portion at the end of the shank that fits into a driving slot of the drill holder on the spindle of the machine

Body: The body of the drill extends from the shank

to the point, and contains the flutes During sharpening, it

is the body of the drill that is

partially ground away

Point: The point is the cutting end of

the drill

Flutes: Flutes are grooves that are

cut or formed in the body of the drill to allow fluids to reach the point and chips

to reach the workpiece surface Although straight flutes are used in some cases, they are normally helical

Land: The land is the remainder of

the outside of the drill body after the flutes are cut The land is cut back somewhat from the outside drill diame-ter in order to provide clearance

Margin: The margin is a short

por-tion of the land not cut away for clear-ance It preserves the full drill diameter

Web: The web is the central portion

of the drill body that connects the lands

Chisel Edge: The edge ground on

the tool point along the web is called the chisel edge It connects the cutting lips

Lips: The lips are the primary

cut-ting edges of the drill They extend from the chisel point to the periphery of the drill

Axis: The axis of the drill is the

cen-terline of the tool It runs through the web and is perpendicular to the diameter

Neck: Some drills are made with a

relieved portion between the body and the shank This is called the drill neck

In addition to the above terms that define the various parts of the drill, there are a number of terms that apply to the dimensions of the drill, including the important drill angles Among these terms are the following:

Length: Along with its outside diameter, the axial length of a drill is listed when the drill size is given In addition, shank length, flute length, and neck length are often used.(see Fig 8.4)

Body Diameter Clearance: The

height of the step from the margin to the land is called the body diameter clear-ance

FIGURE 8.3: Many of the drills used in industry are

shown here and described in this chapter (Courtesy

Cleveland Twist Drill Greenfield Industries)

FIGURE 8.4: Nomenclature of a twist drill shown with taper and tang drives.

Taper shank

Tang Tang drive

Neck

Shank diameter Axis

Straight shank

Land width

Point angle Lip relief angle

Helix angle

Drill diameter Clearance Diameter Body Diameter Clearance Chisel Edge Angle

Shank length

Overall length

Flutes

Flute length

Margin Lip Web Chisel edge

Land

Body

Web Thickness: The web thickness

is the smallest dimension across the

web It is measured at the point unless

otherwise noted Web thickness will

often increase in going up the body

away from the point, and it may have to

be ground down during sharpening to

reduce the size of the chisel edge This

process is called ‘web thinning’ Web

thinning is shown in Figure 8.13

Helix Angle: The angle that the

lead-ing edge of the land makes with the drill

axis is called the helix angle Drills

with various helix angles are available

for different operational requirements

Point Angle: The included angle

between the drill lips is called the point

angle It is varied for different

work-piece materials

Lip Relief Angle: Corresponding to

the usual relief angles found on other

tools is the lip relief angle It is

mea-sured at the periphery

Chisel Edge Angle: The chisel edge

angle is the angle between the lip and

the chisel edge, as seen from the end of

the drill

It is apparent from these partial lists

of terms that many different drill

geometries are possible

8.3 Classes of Drills

There are different classes of drills for

different types of operations

Workpiece materials may also influence

the class of drill used, but it usually

determines the point geometry rather

than the general type of drill best suited

for the job It has already been noted that the twist drill is the most important class Within the general class of twist drills there are a number of drill types made for different kinds of operations

Many of the special drills discussed below are shown in Figure 8.5

High Helix Drills: This drill has a

high helix angle, which improves cut-ting efficiency but weakens the drill body It is used for cutting softer metals and other low strength materials

Low Helix Drills: A lower than

nor-mal helix angle is sometimes useful to prevent the tool from ‘running ahead’ or

‘grabbing’ when drilling brass and sim-ilar materials

Heavy-duty Drills: Drills subject to

severe stresses can be made stronger by such methods as increasing the web thickness

Left Hand Drills: Standard twist drills can be made as left hand tools

These are used in multiple drill heads where the head design is simplified by allowing the spindle to rotate in differ-ent directions

Straight Flute Drills: Straight flute

drills are an extreme case of low helix drills They are used for drilling brass and sheet metal

Crankshaft Drills: Drills that are especially designed for crankshaft work have been found to be useful for machining deep holes in tough materi-als They have a heavy web and helix angle that is somewhat higher than nor-mal The heavy web prompted the use

of a specially notched chisel edge that has proven useful on other jobs as well The crankshaft drill is an example of a special drill that has found wider appli-cation than originally anticipated and has become standard

Extension Drills: The extension drill has a long, tempered shank to allow drilling in surfaces that are nor-mally inaccessible

Extra-length Drills: For deep holes,

the standard long drill may not suffice, and a longer bodied drill is required

Step Drill: Two or more diameters

may be ground on a twist drill to pro-duce a hole with stepped diameters

Subland Drill: The subland or multi-cut drill does the same job as the step drill It has separate lands running the full body length for each diameter, whereas the step drill uses one land A subland drill looks like two drills

twist-ed together

Solid Carbide Drills: For drilling small holes in light alloys and non-metallic materials, solid carbide rods may be ground to standard drill geome-try Light cuts without shock must be taken because carbide is quite brittle

Carbide Tipped Drills: Carbide tips

may be used on twist drills to make the edges more wear resistant at higher speeds Smaller helix angles and

thick-er webs are often used to improve the rigidity of these drills, which helps to preserve the carbide Carbide tipped drills are widely used for hard, abrasive non-metallic materials such as masonry

Oil Hole Drills: Small holes through

the lands, or small tubes in slots milled

in the lands, can be used to force oil under pressure to the tool point These drills are especially useful for drilling deep holes in tough materials

Flat Drills: Flat bars may be ground

with a conventional drill point at the end This gives very large chip spaces, but no helix Their major application is for drilling railroad track

Three and Four Fluted Drills:

There are drills with three or four flutes which resemble standard twist drills except that they have no chisel edge They are used for enlarging holes that have been previously drilled or punched These drills are used because they give better productivity, accuracy, and surface finish than a standard drill would provide on the same job

Drill and Countersink: A

combina-tion drill and countersink is a useful tool

FIGURE 8.5: Special drills are used for some drilling operations.

(a) Jobber s drill

(b) Low-helix drill

(c) High-helix drill

(d) Straight-shank oil-hole drill

(e) Screw-machine drill

(f) Three-flute core drill (g) Left-hand drill (h) Straight-flute drill (i) Step drill (j) Subland drill

for machining ‘center holes’ on bars to

be turned or ground between centers

The end of this tool resembles a

stan-dard drill The countersink starts a short

distance back on the body

A double-ended combination drill

and countersink, also called a center

drill, is shown in Figure 8.6

8.4 Related Drilling Operations

Several operations are related to

drilling In the following list, most of

the operations follow drilling except for

centering and spotfacing which precede

drilling A hole must be made first by

drilling and then the hole is modified by

one of the other operations Some of

these operations are described here and

illustrated in Figure 8.7

Reaming: A reamer is used to

enlarge a previously drilled hole, to

pro-vide a higher tolerance and to improve

the surface finish of the hole

Tapping: A tap is used to provide

internal threads on a previously drilled hole

Reaming and tapping are more involved and complicated than counterboring, countersinking, centering, and spot facing, and are therefore discussed in Chapter 11

Counterboring: Counterboring

produces a larger step in a hole to allow a bolt head to be seated

below the part surface

Countersinking: Countersinking is

similar to counterboring except that the step is angular to allow flat-head screws

to be seated below the surface Counterboring tools are shown in Figure 8.8a, and a counter- sinking tool with two machined holes is shown in Figure 8.8b

Centering: Center drilling is used for accurately locating a hole to be drilled afterwards

Spotfacing: Spotfacing is used to provide a flat-machined surface on a part

8.5 Operating Conditions The varying conditions, under which drills are used, make it difficult to give set rules for speeds and feeds Drill manufacturers and a variety of reference texts provide recommendations for proper speeds and feeds for drilling a variety of materials General drilling speeds and feeds will be discussed here and some examples will be given

Drilling Speed: Cutting speed may

be referred to as the rate that a point on

a circumference of a drill will travel in 1 minute It is expressed in surface feet per minute (SFPM) Cutting speed is one of the most important factors that determine the life of a drill If the cut-ting speed is too slow, the drill might chip or break A cutting speed that is too fast rapidly dulls the cutting lips Cutting speeds depend on the following seven variables:

• The type of material being drilled The harder the material, the slower the cutting speed

• The cutting tool material and

diame-FIGURE 8.6: A double-ended combination drill

and countersink, also called a center drill.

(Courtesy Morse Cutting Tools)

FIGURE 8.7: Related drilling operations: (a) reaming, (b) tapping, (c) counterboring,

(d) countersinking, (e) centering, (f) spotfacing.

FIGURE 8.8: Counterboring tools (a) and countersinking operation (b) are shown here (Courtesy The Weldon Tool Co.)

(a) (b) (c)

(d) (e) (f)

ter The harder the cutting tool

al, the faster it can machine the

materi-al The larger the drill, the slower the

drill must revolve

• The types and use of cutting fluids

allow an increase in cutting speed

• The rigidity of the drill press

• The rigidity of the drill (the shorter

the drill, the better)

• The rigidity of the work setup

• The quality of the hole to be drilled

Each variable should be considered

prior to drilling a hole Each variable is

important, but the work material and its

cutting speed are the most important

factors To calculate the revolutions per

minute (RPM) rate of a drill, the

diame-ter of the drill and the cutting speed of

the material must be considered

The formula normally used to

calcu-late cutting speed is as follows:

SFPM = (Drill Circumference) x (RPM)

Where:

SFPM = surface feet per minute, or

the distance traveled by a point on

the drill periphery in feet each

minute

Drill Circumference = the distance

around the drill periphery in feet

RPM = revolutions per minute

In the case of a drill, the

circumfer-ence is:

Drill Circumference =

Pi/12 x (d) = 262 x d

Where:

Drill Circumference = the distance

around the drill periphery in feet

Pi = is a constant of 3.1416

d = the drill diameter in inches

By substituting for the drill

circum-ference, the cutting speed can now be

written as:

SFPM = 262 x d x RPM

This formula can be used to

deter-mine the cutting speed at the periphery

of any rotating drill

For example: Given a 25 inch drill,

what is the cutting speed (SFPM)

drilling cast iron at 5000 RPM?

SFPM = 262 x d x RPM

SFPM = 262 x 25 x 5000

Answer = 327.5 or 327 SFPM

RPM can be calculated as follows:

Given a 75 inch drill, what is the RPM drilling low carbon steel at 400 SFPM?

RPM = = =

.262 x d 262 x 75 1965 Answer = 2035.62 or 2036 RPM

Drilling Feed: Once the cutting speed has been selected for a particular workpiece material and condition, the appropriate feed rate must be estab-lished Drilling feed rates are selected

to maximize productivity while main-taining chip control Feed in drilling operations is expressed in inches per revolution, or IPR, which is the distance the drill moves in inches for each revo-lution of the drill The feed may also be expressed as the distance traveled by the drill in a single minute, or IPM (inches per minute), which is the product of the RPM and IPR of the drill It can be cal-culated as follows:

IPM = IPR x RPM Where:

IPM = inches per minute IPR = inches per revolution RPM = revolutions per minute

For example: To maintain a 015 IPR feed rate on the 75 inch drill discussed above, what would the IPM feed rate be?

IPM = PR x RPM IPM = 015 x 2036 Answer = 30.54 or 31 IPM

The selection of drilling speed (SFPM) and drilling feed (IPR) for var-ious materials to be machined often starts with recommendations in the

form of application tables from manu-facturers or by consulting reference books

8.5.1 Twist Drill Wear Drills wear starts as soon as cutting begins and instead of progressing at a constant rate, the wear accelerates con-tinuously Wear starts at the sharp cor-ners of the cutting edges and, at the same time, works its way along the cut-ting edges to the chisel edge and up the drill margins As wear progresses, clearance is reduced The resulting rub-bing causes more heat, which in turn causes faster wear

Wear lands behind the cutting edges are not the best indicators of wear, since they depend on the lip relief angle The wear on the drill margins actually deter-mines the degree of wear and is not nearly as obvious as wear lands When the corners of the drill are rounded off, the drill has been damaged more than is readily apparent Quite possibly the drill appeared to be working properly even while it was wearing The margins could be worn in a taper as far back as

an inch from the point To restore the tool to new condition, the worn area must be removed Because of the accelerating nature of wear, the number

of holes per inch of drill can sometimes

be doubled by reducing, by 25 percent, the number of holes drilled per grind 8.5.2 Drill Point Grinding

It has been estimated that about 90 per-cent of drilling troubles are due to improper grinding of the drill point Therefore, it is important that care be taken when resharpening drills A good drill point will have: both lips at the same angle to the axis of the drill; both lips the same length; correct clearance angle; and correct thickness of web

FIGURE 8.9: The included lip angle varies between 90 and 135 degrees (a): two drill points are shown in (b) (Courtesy Cleveland Twist Drill Greenfield Industries)

C 2

C C 2

Lip Angle and Lip Length: When

grinding the two cutting edges they

should be equal in length and have the

same angle with the axis of the drill as

shown in Figure 8.9a Figure 8.9b

shows two ground drill points

For drilling hard or alloy steels, angle

C (Fig 8.9a) should be 135 degrees

For soft materials and for general

pur-poses, angle C should be 118 degrees

For aluminum, angle C should be 90

degrees

If lips are not ground at the same

angle with the axis, the drill will be

sub-jected to an abnormal strain, because

only one lip comes in contact with the

work This will result in unnecessary

breakage and also cause the drill to dull

quickly A drill so sharpened will drill

an oversized hole When the point is

ground with equal angles, but has lips of

different lengths, a condition as shown

in Figure 8.10a is produced

A drill having cutting lips of different

angles, and of unequal lengths, will be

laboring under the severe conditions

shown in Figure 8.10b

Lip Clearance Angle: The clearance

angle, or ‘backing-off’ of the point, is

the next important thing to consider

When drilling steel this angle A (Fig

8.11a) should be from 6 to 9 degrees

For soft cast iron and other soft mate-rials, angle A may

be increased to 12 degrees (or even 15 degrees in some cases)

This clearance angle should increase gradually

as the center of the drill is approached The amount of clearance at the center of the drill determines the chisel point angle B (Fig 8.11b)

The correct com-bination of clearance and chisel point angles should be as follows: When angle

A is made to be 12 degrees for soft materials, angle B should be made approximately 135 degrees; when angle

A is 6 to 9 degrees for harder materials, angle B should be

115 to 125 degrees

While insufficient clearance at the cen-ter is the cause of drills splitting up the web, too much clearance at this point will cause the cutting edges to chip

In order to maintain the necessary accuracy of point angles, lip lengths, lip clearance angle, and chisel edge angle,

the use of machine point grinding is rec-ommended There are many commer-cial drill point grinders available today, which will make the accurate repointing

of drills much easier Tool and cutter grinders such as the one shown in Figure 8.12 are often used

Twist Drill Web Thinning: The tapered web drill is the most common type manufactured The web thickness increases as this type of drill is resharp-ened This requires an operation called web thinning to restore the tool’s origi-nal web thickness Without the web thinning process, more thrust would be required to drill, resulting in additional generated heat and reduced tool life Figure 8.13 illustrates a standard drill before and after the web thinning process Thinning is accomplished with

a radiused wheel and should be done so the thinned section tapers gradually

(a) (b)

(a) (b)

Roll type Dub type Notch type

Original chisel edge

Chisel edge after drill has been shortened

FIGURE 8.10: Drill with equal lip angle but unequal

lip length (a), and drill with unequal lip angle and

unequal lip length (b).

FIGURE 8.12: Tool and cutter grinders, are used to properly sharpen drills and other cutting tools (Courtesy K O Lee Co.)

FIGURE 8.13: Web thinning restores proper web thickness after sharpening twist drills; three methods are shown.

FIGURE 8.11: Drill lip clearance angle (a) and drill chisel point angle (b).

from the point This prevents a blunt

wedge from being formed that would be

detrimental to chip flow Thinning can

be done by hand, but since point

cen-trality is important, thinning by machine

is recommended

8.6 Spade Drills

The tool generally consists of a cutting

blade secured in a fluted holder (See

Figure 8.14) Spade drills can machine

much larger holes (up to 15 in in

diam-eter) than twist drills Spade drills

usu-ally are not available in diameters

smaller than 0.75 inch The drilling

depth capacity of spade drills, with

length-to-diameter ratios over 100 to 1

possible, far exceeds that of twist drills

At the same time, because of their much

greater feed capability, the penetration

rates for spade drills exceed those of

twist drills by 60 to 100 percent

However, hole finish generally suffers

because of this Compared to twist

drills, spade drills are much more

resis-tant to chatter under heavy feeds once

they are fully engaged with the

work-piece Hole straightness is generally

improved (with comparable size

capa-bility) by using a spade drill However,

these advantages can only be gained by

using drilling machines of suitable

capability and power

The spade drill is also a very

eco-nomical drill due to its diameter

flexi-bility A single holder will

accommo-date many blade diameters as shown in

Figure 8.14 Therefore, when

a diameter change is required, only the blade needs to be purchased which

is far less expensive than buying an entire drill

8.6.1 Spade Drill Blades The design of spade drill blades varies with the manu-facturer and the intended application The most com-mon design is shown in Figure 8.15 The locator length is ground to a precision dimen-sion that, in conjunction with the ground thickness of the blade, precisely locates the blade in its holder When the seating pads properly contact the holder, the holes in the blade and holder are aligned and the assembly can be secured with a screw

The blade itself as shown in Figure 8.15, possesses all the cutting geometry necessary The point angle is normally

130 degrees but may vary for special applications In twist drill designs, the helix angle generally determines the cutting rake angle but since spade drills have no helix, the rake surface must be ground into the blade at the cutting edge angle that produces the proper web thickness The cutting edge clearance angle is a constant type of relief, gener-ally 6 to 8 degrees After this clearance

is ground, the chip breakers are ground, about 0.025 inch deep, in the cutting

edge

These chip breakers are nec-essary on spade drill blades and not optional as with twist drills

These notches make the chips narrow enough to flush around the holder Depending on the feed rate, the grooves can also cause a rib to form in the chip

The rib stiffens the chip and causes it to fracture or break more easily which results in shorter, more easily removed chips Margins on the blade act

as bearing surfaces once the tool is in a bushing or in the hole being drilled The width

of the margins will vary from 1/16 to 3/16 inches, depending

on the tool size A slight back taper of 0.004 to 0.006 inch is normally provided and outside diameter clearance angles are generally 10 degrees

8.6.2 Spade Drill Blade Holders The blade holder makes up the major part of the spade drill The blade

hold-er is made of heat-treated alloy steel and

is designed to hold a variety of blades in

a certain size range as shown in Figure 8.14 Two straight chip channels or flutes are provided for chip ejection The holder shank designs are avail-able in straight, Morse taper, and vari-ous other designs to fit the machine spindles The holders are generally sup-plied with internal coolant passages to ensure that coolant reaches the cutting edges and to aid chip ejection

When hole position is extremely crit-ical and requires the use of a starting bushing, holders with guide strips are available These strips are ground to fit closely with the starting bushing to sup-port the tool until it is fully engaged in the workpiece The strips may also be ground to just below the drill diameter

to support the tool in the hole when the set-up lacks rigidity

8.6.3 Spade Drill Feeds and Speeds The cutting speed for spade drills is generally 20 percent less than for twist drills However, the spade drill feed capacity can be twice that of twist drills The manufacturers of spade drills and other reference book publishers provide excellent recommendations for machin-ing rates in a large variety of metals These published rates should generally

be observed Spade drills work best under moderate speed and heavy feed Feeding too lightly will result in either long, stringy chips or chips reduced almost to a powder The drill cutting edges will chip and burn because of the absence of the thick, heat absorbing, C-shaped chips Chips can possibly jam

FIGURE 8.14: Spade drills with various cutting

blades (Courtesy Kennametal Inc.)

FIGURE 8.15: Spade drill cutting blades shows geometry specifications.

Seating pads

Rake surface

Chip breakers

Margin

Blade thickness

Chisel edge Web Back taper Point angle O.D Clearance Locator

length

and pack, which can break the tool or

the workpiece If the machine cannot

supply the required thrust to maintain

the proper feed without severe

deflec-tion, a change in tool or machine may

be necessary

8.7 Indexable Carbide Drills

Indexable drilling has become so

effi-cient and cost effective that in many

cases it is less expensive to drill the hole

rather than to cast or forge it Basically,

the indexable drill is a two fluted, center

cutting tool with indexable carbide

inserts Indexable drills were introduced using square inserts (see Fig 8.16) Shown in Figure 8.17a are indexable drills using the more popular Trigon Insert (see Fig 8.17b) In most cases two inserts are used, but as size increases, more inserts are added with as many as eight inserts in very large tools Figure 8.18 shows six inserts being used

Indexable drills have the prob-lem of zero cutting speed at the center even though speeds can exceed 1000 SFPM at the outer-most inserts Because speed gen-erally replaces feed to some degree, thrust forces are usually

25 to 30 percent of those required

by conventional tools of the same size Indexable drills have a shank, body, and multi-edged point The shank designs gener-ally available are straight, tapered and number 50 V-flange

The bodies have two flutes that are normally straight but may be helical

Because no margins are present to pro-vide bearing support, the tools must rely

on their inherent stiffness and on the balance in the cutting forces to maintain accurate hole size and straightness

Therefore, these tools are usually

limit-ed to length-to-diameter ratios of approximately 4 to 1

The drill point is made of pocketed carbide inserts These inserts are

usual-ly specialusual-ly designed The cutting rake can be negative, neutral, or positive,

depending on holder and insert design Coated and uncoated carbide grades are available for drilling a wide variety of work materials Drills are sometimes combined with indexable or replaceable inserts to perform more than one opera-tion, such as drilling, counterboring, and countersinking

As shown in Figure 8.19a and Figure 8.19b, body mounted insert tooling can perform multiple operations More examples will be shown and discussed

in Chapter 10: Boring Operations and Machines

The overall geometry of the cutting

edges is important to the performance of indexable drills As mentioned earlier, there are no support-ing margins to keep these tools on line, so the forces required to move the cutting edges through the work material must be balanced to minimize tool deflection, partic-ularly on starting, and

to maintain hole size While they are principally designed for drilling, some indexable drills, as shown in Figure 8.20, can perform facing, and boring in lathe

FIGURE 8.18: Indexable drill using six Trigon inserts for drilling large holes (Courtesy Kennametal Inc.)

FIGURE 8.16: Indexing drills were introduced

using square inserts; three sizes are shown here.

(Courtesy Kennametal Inc.)

12¡

84¡

156¡

(b)

FIGURE 8.17: (a) Indexable drills using Trigon inserts (b) A Trigon insert and holder (Courtesy Komet of

America, Inc.)

(a)

applications How well these tools

per-form in these applications depends on

their size, rigidity, and design

8.7.1 Indexable Carbide Drill

Operation

When used under the proper conditions,

the performance of indexable drills is

impressive However, the

manufactur-er’s recommendations must be carefully

followed for successful applications

Set-up accuracy and rigidity is most

important to tool life and performance

Chatter will destroy drilling inserts just as

it destroys turning or milling inserts If

the inserts fail when the tool is rotating in

the hole at high speed, the holder and

workpiece will be damaged Even if lack

of rigidity has only a minor effect on tool

life, hole size and finish will be poor The

machine must be powerful, rigid and

capable of high speed Radial drill

press-es do not generally meet the rigidity requirements Heavier lathes, horizontal boring mills, and N/C machining centers are usually suitable

When installing the tool in the machine, the same good practice fol-lowed for other drill types should be observed for indexable drills The shanks must be clean and free from burrs to ensure good holding and to minimize runout Runout in indexable drilling is dramatically amplified because of the high operating speeds and high penetration rates

When indexing the inserts is neces-sary, make sure that the pockets are clean and undamaged A small speck of dirt or chip, or a burr will cause stress in the carbide insert and result in a micro-scopic crack, which in turn, will lead to early insert failure

8.7.2 Indexable Drill Feeds and Speeds

Indexable drills are very sensitive to machining rates and work materials The feed and speed ranges for various materials, as recommended by some manufacturers of these tools, can be very broad and vague, but can be used

as starting points in determining exact feed and speed rates Choosing the cor-rect feed and speed rates, as well as selecting the proper insert style and grade, requires some experimentation Chip formation is a critical factor and must be correct

In general, soft low carbon steel calls for high speed (650 SFPM or more), and low feed (0.004/0.006 IPR) Medium and high carbon steels, as well

as cast iron, usually react best to lower speed and higher feed The exact speed and feed settings must be consistent with machine and set-up conditions, hole size and finish requirements, and chip formation for the particular job 8.8 Trepanning

In trepanning the cutting tool produces a hole by removing a disk shaped piece also called slug or core, usually from flat plates A hole is produced without reducing all the material removed to chips, as is the case in drilling The trepanning process can be used to make disks up to 6 in in diameter from flat sheet or plate A trepanning tool also called a “Rotabroach” with a core or slug is shown in Figure 8.21a and an end view of a Rotabroach is shown in Figure 8.21 b

Trepanning can be done on lathes, drill presses, and milling machines, as well as other machines using single point or multi point tools Figure 8.22 shows a Rotabroach cutter machining holes through both sides of a rectangu-lar tube on a vertical milling machine Rotabroach drills provide greater tool life because they have more teeth than conventional drilling tools Since more teeth are engaged in the workpiece, the material cut per hole is distributed over

a greater number of cutting edges Each cutting edge cuts less material for

a given hole This extends tool life sig-nificantly

Conventional drills must contend with a dead center area that is prone to chip, thus reducing tool life In the chis-el-edge region of a conventional drill the cutting speed approaches zero This

FIGURE 8.19: Body-mounted insert

tool-ing can perform multiple operations.

(Courtesy Komet of America, Inc.)

FIGURE 8.20: In addition to drilling, indexable drills can perform boring and facing

operations.

(b)

(a)

is quite different from the speed at the

drill O.D Likewise, thrust forces are

high due to the point geometry

Rotabroach drills cut in the region from

the slug O.D to the drill O.D Since

only a small kerf is machined, cutting

speeds are not so different across the

face of a tooth This feature extends

tool life and provides uniform

machin-a b i l i t y Figure 8.23 shows drilling holes with conventional drills and hole broaching drills

8.8.1 Trepanning Operations Trepanning is a roughing operation

Finishing work requires a secondary operation using reamers or boring bars

to get a specified size and finish Of the many types of hole-making operations,

it competes with indexable carbide cut-ters and spade drilling

Several types of tools are used to trepan

The most basic is a single or double point cutter (Fig 8.24) It orbits the spindle cen-terline cutting the periphery of the hole

Usually, a pilot drill centers the tool and drives the orbiting cutter like a compass inscribing a circle on paper Single/double point trepanning tools are often adjustable within their working diameter They are efficient and versatile, but do begin to have rigidity problems when cutting large holes

- 6 1/2 inches in diameter is about the max-imum

A hole saw is another tool that trepans holes It is metalcutting’s ver-sion of the familiar doorknob hole cut-ter used in wood Hole saws have more teeth and therefore cut faster than sin-gle, or double-point tools Both hole

saws and single-point tools curl up a chip in the space, or gullet, between the teeth, and carry it with them in the cut

Hole broaching tools are hybrid trepanners (Fig 8.21a and 8.21b) They com-bine spiral flutes like a drill with a broach-like progressive tool geometry that splits the chip so it exits the cut along the flutes

With this design, the

larger number of cutting edges and chip evacuation, combine to reduce the chip load per tooth so this drill can cut at higher feed rates than trepanning tools and hole saws Like the hole saw, a hole broaching tool has a fixed diameter One size fits one hole

8.8.2 Cutting Tool Material Selection M2 High Speed Steel (HSS)

is the standard Rotabroach cutting tool material M2 has the broadest applica-tion range and is the most economical tool material It can be used on ferrous and non-ferrous materials and is gener-ally recommended for cutting materials

up to 275 BHN M2 can be applied to harder materials, but tool life is dramat-ically decreased

TiN coated M2 HSS Rotabroach drills are for higher speeds, more endurance, harder materials or freer cutting action to reduce power consumption The TiN coating reduces friction and operates at cooler temperatures while presenting a harder cutting edge surface Increased cutting speeds of 15 to 25 % are recom-mended to obtain the benefits of this sur-face treatment The reduction in friction and resistance to edge build-up are key benefits The ability to run at higher speeds at less power is helpful for appli-cations where the machine tool is slightly underpowered and TiN coated tools are recommended for these applications TiN coated tools are recommended for applications on materials to 325 BHN Carbide cutting tool materials are

FIGURE 8.21: Trepanning tool also called Rotabroach with

core or slug (Courtesy Hougen Manufacturing, Inc.)

FIGURE 8.23: Drilling holes with conventional drill and hole

broaching drill Surface speed increases with distance from

center.

FIGURE 8.24: Traditional trepanning tool orbits around a center drill.

FIGURE 8.22: Rotabroach machining

set-up on a milling machine (Courtesy

Hougen Manufacturing, Inc.)

Velocity Approaches

"Zero" at

Center Point

Velocity of Cutting Edge (SFPM) Kerf

Hole Broaching Drill Conventional Drill

(a)

(b)